1. Field of the Invention
The subject invention generally pertains to electronic power conversion circuits, and more specifically to high frequency switched mode power conversion electronic circuits with active power factor correction.
2. Description of Related Art
Power factor correction electronic circuits are useful for reducing line current harmonics. These circuits improve power quality and provides a friendlier interface between the power utility and electronic apparatus of all sorts. The ideal power line interface is a resistor. With a resistor load to the power line interface the line current is directly proportional to the line voltage. With many electronic circuits connected to the power line the input is a rectifier or rectifier bridge with a capacitor line frequency filter. For most of the line frequency cycle the diodes do not conduct and the line current is zero since the diodes are reverse biased by the line filter capacitor. When the diodes become forward biased it is only for a very small fraction of the line cycle when the currents are very large, typically four to ten times larger than the average current. With this rectifier and capacitor arrangement the line current is zero for most of the line cycle and the line current wave form consists of short sharp high magnitude pulses, occurring typically twice per line cycle. These current pulses contain high order harmonics of the line frequency and provide a difficult and uneven interface to the power grid. Power factor is one measure of how close the current wave form is to a sine wave. Power factor correction refers to any method that reduces the harmonics of the line current wave form resulting in a line current wave form that more closely resembles a pure sine wave. Some countries are now mandating that line current harmonics of electronic apparatus be limited. One method of power factor correction is to place a line frequency low pass filter at the input to the power converter. Such a filter would pass the line frequency fundamental component and reject harmonics of the line frequency. A line frequency filter is seldom the choice for reducing line frequency harmonics because of the size and weight of a suitable line frequency filter. There are power conversion circuits that accomplish active power factor correction by regulating the input current so that the input current is proportional to the input voltage. By modulating the duty cycle of the converter to force the input current to be proportional to the input voltage the circuit appears as a resistor to the utility interface. Active power factor correction circuits are becoming prolific because they regulate line current and thereby eliminate line current harmonics which would exist without the line current regulation circuits, thereby enabling manufacturers of electronic apparatus to comply with the legal mandates for line current harmonics. The power converter circuits that regulate line current, called power factor rectifiers by those skilled in the art of power conversion, also regulate output voltage, but the output voltage regulation is poor. The control circuit forces the input current to be proportional to line voltage and only uses the output voltage error to set the constant of proportionality. Typically the current regulation control loop of a power factor rectifier is fast and can respond quickly to line voltage changes to correct the input current, but the output voltage regulation loop is much slower. The bandwidth of the outer voltage control loop is much less than the line ripple frequency so that the output voltage regulation is poor and contains a large line ripple frequency component. By increasing the bandwidth of the output voltage control loop, the control of the input current is reduced, so that there is a trade off between output voltage control and input current control. It is impossible to obtain precise control of both output voltage and input current with a single power conversion stage. If both the input current must be precisely regulated and the output voltage must be precisely regulated then a second stage of power conversion is required. The second stage of power conversion will typically follow the first power factor rectifier stage in a series connection where each stage processes all of the power, i.e., all of the load power must pass through each power converter stage and the output power of the first stage is the input power to the second stage. An example of a typical power converter system with active power factor correction (regulated input current) and precisely regulated output voltage is shown in FIG. 1. In FIG. 1 the first converter, a power factor rectifier, precisely regulates line current and loosely regulates its output voltage and the second power converter precisely regulates output voltage. The power factor rectifier consists of a diode rectifier circuit which changes the AC line voltage to a pulsating DC voltage and a suitable DC to DC converter. In general, a rectifier refers to a circuit that changes AC into DC. A power factor rectifier is a circuit that changes AC into DC and also provides a regulated line current with low line frequency harmonic content. One example of a power factor rectifier is shown in FIG. 2. Wave forms for the FIG. 2 circuit are shown in FIG. 3. The circuit shown in FIG. 2 is a full wave bridge diode rectifier and a boost converter. The boost converter is the most common converter choice for a power factor rectifier, but not the only choice. The circuit shown in FIG. 2 is almost identical to FIG. 19.7 in Solid State Power Conversion Handbook by Ralph Tarter. Many other examples of power factor rectifiers are found in the publications of the IEEE Power Electronics Society, such as the proceedings of the Power Electronics Specialists Conference, the Applied Power Electronics Conference, and the IEEE Transactions on Power Electronics. All of these conference proceedings for the last decade or more will have several papers on power factor rectification topics. Other examples of topologies for DC to DC converters used in power factor rectifiers include the flyback converter, the buck boost converter, the SEPIC converter, and the Cuk converter. Any DC to DC converter that has the capability of delivering power to its output down to, but not including, zero input voltage is a suitable alternative to the boost converter, shown in FIG. 2. The control block of FIG. 2 modulates the duty cycle of Q1 to provide fast current regulation to force the input current, I.sub.RS, as sensed by R.sub.S to be proportional to the input voltage, which is V.sub.A -V.sub.B. See FIG. 3. Since the input current is proportional to the input voltage the power factor rectifier appears as a resistor to the line and the line current harmonics are largely eliminated. The control block also senses the output voltage, but the response to output voltage variations is slow with a control loop bandwidth much less than the output voltage ripple frequency. For a 60 hertz line frequency the circuit shown will have a substantial 120 hertz output ripple component, as illustrated in FIG. 3, since the line frequency is effectively doubled by the diode rectifier circuit. The output voltage for the FIG. 2 circuit is typically about 375 volts for an AC mains line input voltage range of 90 to 264 volts rms. The line frequency ripple magnitude will be load dependent and may be in the range of 10% or approximately 40 volts peak to peak for the maximum load. The large ripple voltage variation at the output of the boost power factor rectifier is not acceptable for many applications so a second stage of regulation that provides precise output voltage regulation is typically added. FIG. 4 illustrates a simple DC to DC converter representative of the second block of FIG. 1. FIG. 4 illustrates a buck converter which contains a control circuit that sets the duty cycle of Q2. The control circuit of FIG. 4 senses the output voltage and precisely regulates the output voltage. The bandwidth of the outer voltage loop of the control circuit must be significantly higher than the line frequency ripple or the control loop must contain an input voltage feed forward mechanism that rejects the line frequency input voltage variations. These features are typical of DC to DC converters used for the purpose of output voltage regulation.
The prior art illustrated shows a series connection of a power factor rectifier and a DC to DC converter, each of which processes all of the power that is ultimately delivered to the load. The power processed by the boost converter contained in the power factor rectifier of FIGS. 1 and 2 is equal to the average voltage at the input of the boost converter multiplied by the average current delivered to the boost converter at its input or the average input power, which is also equal to the average voltage at the output of the boost converter multiplied by the average current delivered by the boost converter at its output divided by the efficiency of the boost converter. For the boost converter the power processed is slightly greater than the total power delivered to the load, or one can say simply that all of the load power is processed by the boost converter. For the second stage DC to DC converter of FIGS. 1 and 4 one can also say that all of the load power is processed by the second stage DC to DC converter. The power processed by the DC to DC converter will be equal to the average voltage at the input to the DC to DC converter multiplied by the average current delivered to the input of the DC to DC converter or the average input power, or alternately, it is the average voltage supplied at the output of the DC converter multiplied by the average current delivered to a load at the output of the DC to DC converter, divided by the efficiency of the converter. If the load power is 1 kilowatt then we can say that the prior art system consists of two 1 kilowatt power converters placed in series. The 1 kilowatt figure is a measure of both the output power capability and the power processed by the individual converters in the two converter system.
In the scheme illustrated in FIG. 1 all of the power is processed by both sub-converters. The size, weight, and cost of a power conversion circuit are all roughly proportional to the power processed by the power conversion circuit, although there is some dependence of size, weight, and cost on the peak power processed.